Growth of the chicken embryo is directly related to the availability of oxygen (O2) during development. In an effort to understand the relationship between oxygen availability, embryonic oxygenation and embryonic growth the proposed research will test the hypothesis that the chicken embryo incubated continuously in an elevated oxygen concentration develops unique adaptations which permit it to thrive in hyperoxia and which protect it from oxygen toxicity. The objectives are: 1) To demonstrate the presence or absence of embryonic adaptation to hyperoxia. Responses of normoxic embryos (incubated in air) and hyperoxic embryos (incubated in 40% or 70% O2) to acute increases (relative hyperoxia) and acute decreases (relative hypoxia) in ambient O2 will be compared. Physiological responses to be studied include hemodynamics (blood pressure and heart rate), oxygen consumption and carbon dioxide production, acid-base and blood gas status, and embryonic growth rate. 2) To gauge the effectiveness of embryonic adaptation to hyperoxia by comparing the degree of lipid peroxidation (LPO), a principal sign of oxygen toxicity, in normoxic and hyperoxic embryos. The thiobarbituric acid test will be used as an index of LPO. 3) To begin analysis of the mechanism by which embryonic adaptation to hyperoxia occurs, or alternatively, the factors which limit adaptation. Tissue oxygen tensions in normoxic and hyperoxic embryos will be compared to determine whether they are maintained within normal limits despite chronic ambient hyperoxia. The levels of protective antioxidant enzymes (superoxide dismutase, catalase, glutathione peroxidase) and the rate of oxygen radical production will be compared in normoxic and hyperoxic embryos. The embryo, despite its physiological and biochemical plasticity, has previously been overlooked in terms of its capacity to adjust to alterations in oxygen availability. The avian embryo is uniquely suited to this research by virtue of its independence from maternal intervention.